This invention relates to herbicidal compositions and methods of use. In particular, this invention relates to herbicidal compositions comprising an herbicidally active thiocarbamate in combination with an organophosphorus compound, the latter serving to prolong the effectiveness of a single application of the thiocarbamate herbicide in controlling undesired plant growth.
Thiocarbamates are well known in the agricultural art as herbicides useful for weed control in crops such as corn, potatoes, beans, beets, spinach, tobacco, tomatoes, alfalfa, and others. Thiocarbamates are primarily used in pre-emergence application. They have been found to be particularly effective when incorporated into the soil prior to the planting of the crop. As a herbicide, the thiocarbamate is most concentrated immediately after its application. How long thereafter the initial concentration is retained depends in large part on the particular soil used. Thus the rate at which the thiocarbamate herbicide concentration declines following its application tends to vary from one type of soil to the next. This is evident both in the observable extent of actual weed control and in the detectable presence of undegraded thiocarbamate remaining in the soil after considerable time has elapsed.
It has now been discovered that the soil persistence of certain herbicidally active thiocarbamates is significantly extended by the incorporation of certain organophosphorus compounds, which have no herbicidal activity of their own, into the herbicide formulation. This improvement in the soil persistence of the thiocarbamate can manifest itself in a variety of ways. Improved soil persistence can be shown by herbicidal efficacy tests, wherein the degree of weed injury is measured after a set period of time following application of the herbicide. In such a test, the organophosphorus compound, which has no herbicidal activity of its own, is shown to increase the herbicidal effectiveness of the thiocarbamate by increasing the persistence of the latter in the soil, and thus prolonging its effective life. Also demonstrated herein are soil persistence test results which evidence by periodic soil analyses that the number of thiocarbamate herbicide molecules that degrade in the soil is lessened in the presence of an organophosphorus extender of this invention.